Users of computer networks often want a rich variety of network services while also demanding high-speed data access. But network services take processing time that tends to slow the data delivery rate.
One way to achieve both high-speed and high-performance data packet processing is through parallelism or multi-threading in which network devices execute more than one thread simultaneously. Multi-threading is useful when a single task takes so long to complete that processing packets serially would slow down the overall packet throughput of a system too much.
During multi-threading, data packets sometimes flow into a multi-threaded task in one order (for example, P1, then P2) and flow out of the multi-threaded task in a different order (P2, then P1). This reordering can occur when the task is performed on P1 in one thread more slowly than the performance of the task on packet P2 in another thread. The different speeds of processing can result from the contention between threads for shared resources, such as memory bus access. Packet reordering is undesirable if the two data packets, P1 and P2, are bound for the same host.
Techniques that have been proposed to prevent packet reordering include adding hardware support, using only single-threaded processing, guaranteeing execution time per thread or using barrier or other synchronization mechanisms to ensure sequential processing of packets.